The present invention relates to a method of removing a component or components from a gas stream by adsorption onto a solid adsorbent with regeneration of the adsorbent at intervals.
In such methods, the gas stream is fed in contact with a solid adsorbent to adsorb the component to be removed which gradually builds-up in the adsorbent. The concentration of the removed component in the adsorbent will gradually rise. The concentration of the removed gas component in the adsorbent will not be uniform but will be highest at the upstream end of the adsorbent bed and will tail off progressively through a mass transfer zone in the adsorbent. If the process is conducted indefinitely, the mass transfer zone will progressively move downstream in the adsorbent bed until the component which is to be removed breaks through from the downstream end of the bed. Before this occurs, it is necessary to regenerate the adsorbent.
In the pressure swing adsorption PSA system this is done by stopping the flow into the adsorbent of gas to be treated, depressurising the adsorbent and, usually, by passing through the bed counter-current to the product feed direction a flow of a regenerating gas, usually at a lower pressure than the gas to be treated and low in its content of the component adsorbed on the bed.
As the component which is being removed is adsorbed while the bed is on-line, the adsorption process will generate heat of adsorption causing a heat pulse to progress downstream through the adsorbent. During the regeneration process, heat must be supplied to desorb the gas component which has been adsorbed on the bed. In PSA, one aims to commence regeneration before the heat pulse mentioned above has reached the downstream end of the bed. The direction of the heat pulse is reversed by the process of regeneration and the heat which derived from the adsorption of the gas component in question is used for desorbing that component during regeneration. One thus avoids having to add heat during the regeneration step.
An alternative procedure is known as temperature swing adsorption (TSA). In TSA, the cycle time is extended and the heat pulse mentioned above is allowed to proceed out of the downstream end of the adsorbent bed during the feed or on-line period. To achieve regeneration it is therefore necessary to supply heat to desorb the adsorbed gas component. To this end the regenerating gas used is heated for a period to produce a heat pulse moving through the bed counter-current to the normal feed direction. This flow of heated regenerating gas is usually followed by a flow of cool regenerating gas which continues the displacement of the heat pulse through the bed toward the upstream end. TSA is characterised by an extended cycle time as compared to PSA.
Each procedure has its own characteristic advantages and disadvantages. TSA is energy intensive because of the need to supply heat to the regenerating gas. The temperatures needed for the regenerating gas are typically sufficiently high, e.g. 150.degree. C. to 200.degree. C., as to place demands on the system engineering which increase costs. Typically, there will be more than one unwanted gas component which is removed in the process and generally one or more of these components will adsorb strongly and another much more weakly. The high temperature used for regenerating in TSA needs to be sufficient for the desorption of the more strongly adsorbed component. Usually, in order to deal with the need to adsorb differing components from the gas stream at the same time, the optimised TSA system will use a dual adsorbent bed containing a first layer for adsorbing the more strongly adsorbed component (e.g. water) and a second layer for adsorbing a more weakly adsorbed component (e.g. carbon dioxide). Thus, for removing water and carbon dioxide from the gas stream, a TSA system will typically use an adsorbent bed having a first layer of alumina for the removal of water and a second layer of 13X molecular sieve for the removal of carbon dioxide and other minor components. However, the process usually uses either all zeolite compared to alumina and so needs a high regeneration temperature, normally in excess of 100.degree. C. to virtually boil the adsorbed water off the zeolite. To minimise the amount of water which has to be desorbed, it is usual to pre-cool the air to be treated, thus condensing out much of its water content.
The high temperature used in a TSA system give rise to a need for the use of insulated vessels, a purge preheater and an inlet end precooler and generally the high temperatures impose a more stringent and costly mechanical specification for the system. In operation, there is extra energy cost associated with using the purge preheater.
Whilst the PSA system avoids many of these disadvantages by avoiding the need for coping with high temperatures, the short cycle time which characterises PSA brings its own disadvantages. In each cycle of operation the adsorbent is subjected to a feed period during which adsorption takes place followed by depressurisation, regeneration and repressurisation. During depressurisation, the feed gas in the bed is vented off and lost. The amount of feed-gas lost in this way is known as the "switch loss". The short cycle time in the PSA system gives rise to high switch losses. Also, because the cycle is short it is necessary that the repressurisation be conducted quickly. In practice, one has two beds of adsorbent undergoing the above cycles of operation with the cycles being phased with respect to one another that there is always one of the two beds in its feed or on-line period. Accordingly, the time available for repressurisation and regeneration is limited by the time the other bed can spend in the feed part of its cycle which is in turn limited by the short cycle time. The rapid repressurisation implied by these constraints causes transient variations in the feed and product flows which can adversely affect the plant operation, particularly the operation of processes downstream from the adsorption system.
PSA is described by Skarstrom, C. W. in "Heatless Fractionation of Gases over Solid Adsorbents", Vol. 11, 95, N. W. Li(ED) C.R.C. Press, Cleveland, Ohio 1972 and in U.S. Pat. No.4,711,645 (Kumar).
TSA is described by von Gemmingen, U. in "Designs of Adsorptive driers in air separation plants"--Reports on Technology 54/1994--(Linde) using lower than normal temperatures, i.e. 80.degree. to 130.degree. C. and short cycle times.
A still lower temperature form of TSA is described in U.S. Pat. No. 5,137,548 (Grenier) using a regeneration temperature of 35.degree. C. with a 13X molecular sieve adsorbent. The prior removal of water by cooling the feed air is essential to this process.
U.S. Pat. No. 4,541,851 discloses that one may practise TSA such that the heat pulse is consumed in desorbing both the more strongly and weakly adsorbed components from the adsorbent.
U.S. Pat. No. 4,249,915 and U.S. Pat. No. 4,472,178 disclose an adsorption process in which moisture and carbon and carbon dioxide are removed from atmospheric air by adsorption in separate respective beds. The moisture laden bed is regenerated by pressure swing adsorption in a relatively short operating cycle while the CO.sub.2 laden bed is regenerated thermally at considerably longer time intervals. The accomplishment of this naturally necessitates considerably increased apparatus cost in view of the need for separate columns to contain the moisture and carbon dioxide removing beds and additional ancillary equipment. Whilst providing certain benefits, to some extent the teaching of these specifications suffers from the disadvantages of both PSA and TSA. One has the high switch loss and variable output of PSA water removal module and one also has the high energy demand and equipment cost of the TSA carbon dioxide removal module.
EP-A-0766989 discloses the use of alumina followed by molecular sieve for removing carbon dioxide and water from air prior to cryogenic separation. Here the heat pulse produced by heating the regenerating nitrogen rich gas is not consumed in the bed but is halted before it enters the upstream alumina portion of the bed.
In `Adsorption Purification For Air Separation Units`-M. Grenier et al, Intersociety Cryogenics Symposium, Winter Annual Meeting of ASMI, Dec. 9-14, 1984, New Orleans, La., adsorption of carbon dioxide and water from air is carried out using a bed containing an upstream (having regard to the flow direction during adsorption) portion of alumina and a downstream portion of molecular sieve. Water is adsorbed on the alumina which protects the molecular sieve from the water. Carbon dioxide is adsorbed principally on the molecular sieve.
Regeneration of the adsorbent is achieved by passing heated nitrogen through the adsorbent bed in the direction opposite to the flow direction during adsorption. After a period, the heating of the nitrogen is stopped but the flow of nitrogen is continued. This produces a pulse of heat moving through the adsorbent and the heat in the heat pulse provides energy for desorbing the water and carbon dioxide from the adsorbent. It is indicated that one may seek to arrange for the heat added during the heating of the nitrogen to exactly balance the heat needed for desorption. If the heating is continued for too long, heat will be left in the bed at the end of the regeneration which will interfere with adsorption of water and carbon dioxide when the bed goes back on-line and may be passed into the cryogenic nitrogen/oxygen separator, disturbing its functioning.
It is said that if the heating is discontinued too soon the result will be that part of the bed will not be regenerated. This is seen as being of lesser consequence and so one may provide a very small over-sizing of the alumina bed so that the heat pulse never leaves the bed but dies within the extra alumina during successive cycles.